After installing the mod on a server (World gen mods being Biomes o' Plenty and Terraforged) All the existing biomes besides vanilla ones were changed to different biomes. Below are the before image (from a backup taken before adding the mod) and what the same area looked like after adding the mod. I have also added a screenshot of the world map showing the affected areas I had not reloaded



In the images above the Highland biome was changed to a Fungal Jungle, and not all but many of the snowy biomes were changed to other types of snowy biomes.

Which version are you using?

The version posted just yesterday for 1.16.5

Probably related to #43. Keep track of the issue there.